U.S. Pat. No. 3,658,493 relates that various nitrogen materials of limited solubility in fuel oil, having at least one straight chain hydrocarbon segment of at least 6 carbon atoms, such as amides and ammonia or amine salts, are wax crystal modifiers for distillate fuel oil and can be used with various ethylene backbone polymeric pour point depressants. In one form, the present invention is based on the discovery that various nitrogen materials, including some of those of the aforementioned patent, are very effective cold flow improvers for middle distillate fuel oils when used with certain hydrocarbon materials, particularly waxy hydrocarbon materials such as petrolatums, microcrystalline waxes, wax alkylated naphthalene, or isomerized wax produced by the Friedel-Crafts isomerization of normal paraffin waxes, which materials may still be used in combination with an ethylene backbone pour point depressant or another pour depressant. In another form of the present invention, various nitrogen-containing materials outside the scope of said U.S. Pat. No. 3,658,493 have been found very effective for enhancing the effectiveness of ethylene backbone pour point depressants.